Initial import of either-1.6.1. am: d767123c85 am: f1058920e6 am: 8e5719ab53

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/either/+/1621403

Change-Id: Icbeb27c794b53bb205de4fbb047f25042f7753bc